3.3 Satellite-Borne Multispectral Systems
What are the differences between a camera system and a scanning system? The following are some of the major differences:
´ A rotating mirror is added in front of the lens of camera
´ In a scanning system, films are changed to photo-sensitive detectors and magnetic tapes. They are used to store the collected spectral energy (Figure 3.16).
Figure 3.16. A multispectral scanning system
Landsat Multispectral Scanner System
The first of the Landsat series was launched in 1972. The satellite was called Earth Resources Technology Satellites (ERTS-1). It was later renamed as Landsat - 1. On board of Landsat-1 are two sensing systems: multispectral scanning system (MSS) and return beam vidicon (RBV). RBV was discontinued since Landsat-3. MSS is briefly introduced here because it is still being used. The MSS sensor has 6 detectors per band (Figure 3.17). The scanned radiance is measured in four image bands (Figure 3.18).
Figure 3.17. Each scan will collect six image lines.
Figure 3.18. Four image bands with six detectors in each band.
MSSs have been used on Landsat - 1, 2, 3, 4, 5. They are reliable systems. The spectral region of each band is listed below:
Landsat 1, 2
Landsat 4, 5
0.5 - 0.6 mm
0.6 - 0.7mm
0.7 - 0.8 mm
0.8 - 9.1 mm
Landsat 3 had a short life. The MSS systems on Landsat 3 were modified as compared to Landsat 1 and 2. Landsat-6 was launched unsuccessfully in 1993.
Each scene of MSS image covers 185 km X 185 km in area. It has a spatial resolution of 79 m X 57 m. An advantage of MSS is that it is less expensive. Sometimes one detector is left blank or its signal is much different from other ones, creating banding or striping. We will discuss methods for correcting these problems in Chapter 5.
Landsat Thematic Mapper System
Since the launch of Landsat 4 in 1982, a new type of scanner, called Thematic Mapper (TM), has been introduced. It
´ Increased the number of spectral bands.
´ Improved spatial and spectral resolution.
´ Increased the angle of view from 11.56░ to 14.92░.
0.45 - 0.52 mm
0.52 - 0.60
0.63 - 0.69
0.76 - 0.90
1.55 - 1.75
2.08 - 2.35
10.4 - 12.5 mm
MSS data are collected on only one scanning direction. TM data are collected on both scanning directions (Figure 3.19).
Figure 3.19. Major changes of the TM system as compared to the MSS system.
High Resolution Visible (HRV) Sensors
A French satellite called 'Le SystŔme Pour l'observation de la Terre' (SPOT) (Earth Observation System) was launched in 1986. On board this satellite, a different type of sensors called High Resolution Visible (HRV) were used. The HRV sensors have two modes: the panchromatic (PAN) mode and the multispectral (XS) mode.
The HRV panchromatic sensor has a relatively wide spectral range, 0.51 - 0.73 mm, with a higher spatial resolution of 10 x 10 m2
HRV Multispectral (XS) mode
0.50 - 0.59 mm
0.61 - 0.68 mm
0.79 - 0.89 mm
The spatial resolution for the multispectral (XS) mode is 20 x 20 m2.
Besides the difference of spectral and spatial resolution design from the Landsat sensor systems, major differences between MSS/TM and HRV are the use of linear array (also called pushbroom) detectors and the off-nadir observation capabilities with the HRV sensors (Figure 3.20). Instead of mirror rotation in the MSS or the TM sensors which collect data using only a few detectors, the SPOT HRV sensors use thousands of detectors arranged in arrays called "charge-coupled devices" (CCDs). This has significantly reduced the weight of the sensing system and power requirement.
Figure 3.20. The SPOT HRV systems
A mirror with the view angle of 4.13░ is used to allow ▒27░ off nadir observation. An advantage of the off-nadir viewing capability is that it allows more frequent observations of certain targeted area on the earth and acquisitions of stereo-pair images. A disadvantage of the HRV sensors is the difficulties involved in calibrating thousands of detectors. The radiometric resolution of MSS is 6 to 7 bits, while both TM and HRVs have an 8 bit radiometric resolution.
The orbital cycle is 18 days for Landsats 1 - 3; 16 days for landsats 4, 5; 26 days for SPOT-1 (SPOT HRV sensors can repeat the same target in 3 to 5 days due to their off-nadir observing capabilities).
AVHRR - Advanced Very High Resolution Radiometer
Among many meterological satellites, the Advanced Very High Resolution Radiometers (AVHRR) on board the NOAA series (NOAA-6 through 12) have been widely used. NOAA series were named after the National Oceanic and Atmospheric Administration of the United States.
The AVHRR sensor has 5 spectral channels
0.58 - 0.68 mm
0.72 - 1.10 mm
3.55 - 3.95 mm
10.3 - 11.30 mm
11.5 - 12.50 mm
Swath width 2400 Km
The orbit repeating cycle is twice daily. This is an important feature for frequent monitoring. NOAA AVHRRs have been used for large scale vegetation and sea ice studies at continental and global scales.
Earth Observing System (EOS)
To document and understand global change, NASA initiated Mission to Planet Earth. This is a program involving international efforts to measure the Earth from space and ground. Earth Observing System is a primary component of the Mission to Planet Earth. EOS includes the launch of a series of satellites with advanced sensor systems by the end of this century. Those sensors will be used to measure most of the measurable aspects of the land, ocean and atmosphere, such as cloud, snow, ice, temperature, land productivity, ocean productivity, ocean circulation, atmospheric chemistry, etc.
Among various sensors on board the first six satellites to be launched, there is a sensor called Moderate Resolution Imaging Spectrometer (MODIS). It has 36 narrow spectral bands between 10-360 nm. The spatial resolution changes as the spectral band changes. Two bands have 250 m, 5 have 500 m while the rest have 1000 m resolution. The sensor is planned to provide data covering the entire Earth daily.
Other Satellite Sensors
GOES - Geostationary Operational Environmental Satellite (Visible to NIR, Thermal)
DMSP - Defense Meterological Satellite Program 600 m resolution (Visible to NIR, Thermal) used, for example for urban heat island studies
Nimbus - CZCS - coastal zone color scanner, 825 m spatial resolution
Channels (6 total) Spectral Resolution 1 - 4 0.02 Ám for chlorophyll absorption studies 5 - 6 NIR -thermal
Two private companies, Lockheed, Inc. and Worldview, Inc. are planning to launch their own commecial satellites in 2-3 years time with spatial resolutions ranging from 1 m to 3 m. In Japan, the NASDA (National Space Development Agency) has developed the Marine Observation System (MOS). On board this system, there is a sensor called Multispectral Electronic Self-scanning Radiometer (MESSR) with similar spectral bands as the Landsat MSS systems. However, the spatial resolution of the MESSR system is 50 x 502.
Other countries such as India and the former USSR have also launched Earth resources satellites with different optical sensors.